The invention relates to a method and apparatus for measuring gas pressure in a vessel.
It has long been known that the pressure in a vessel can be measured by the Pirani method, by measuring the electric power obtained at a resistance element, for example a slide wire, which power gives the heat transfer from the resistance element to the vessel wall. Using the known relationship between pressure and thermal conductivity of the gas, it is finally possible to determine the required gas pressure in the vessel from the heat transfer between resistance element and vessel wall.
However, it is also known that there are in this method of measurement various interfering factors which can be eliminated or compensated with difficulty or only with considerable effort. Not only is the thermal conduction by the gas dependent on the temperature of the vessel wall, but the heat transfer between resistance element and vessel wall also contains components which are caused by radiant exchange and thermal conduction in the region of the connections of the resistance element. The two components are also highly dependent on the temperature of the vessel wall, which can be measured only with considerable effort to the extent required for a sufficiently accurate measurement.
Attempts have already been made to compensate the temperature influences by installing a temperature-dependent resistance in a Wheatstone bridge containing the resistance element. However, it is very difficult to achieve satisfactory compensation over a relatively large pressure range in this way.
DE-A-43 08 434 also discloses that, in such a solution, it is possible to measure the resistance value of temperature dependent resistance and to use it for an additional, for example computational temperature compensation. Here too, however, the compensation of the temperature influence is not absolutely optimal in spite of the considerable effort.
It is accordingly the object of the invention to provide a method of measurement in which interfering factors are eliminated in a radical and simultaneously simple manner and in particular the influence of the wall temperature on the value determined for the gas pressure is compensated over a large pressure range.
The invention provides a method in which the gas pressure is determined in a simple manner in a form essentially unimpaired by interference, from readily obtainable electrical variables. Also provided are apparatuses by means of which the method according to the invention can be carried out in a particularly advantageous manner. The proposed embodiments make it possible, depending on the requirements regarding the accuracy of measurement, to carry out a very accurate determination of the gas pressure by means of separate measurement and digital processing of the electrical variables.